The drm_panel_enable/disable and drm_panel_prepare/unprepare calls are
not fine grained enough to abstract all the different steps we need to
take (and VBT sequences we need to exec) properly. So simply remove the
panel _enable/disable and prepare/unprepare callbacks and instead
export intel_dsi_exec_vbt_sequence() from intel_dsi_panel_vbt.c
and call that from intel_dsi_enable/disable().